KAP140 Autopilot Details

"Ron Natalie" wrote:

Depends which model you have. The 20/30 will not intercept a course, just
hold it.

Yes, when I said "those S-Tec autopilots" I was referring specifically to
"those," the 20/30.


The more expensive ones in the line will intercept the course.

The KAP 140 is similar in many respects to the S-Tec 55. Actually the 55 is
a little nicer in several ways. I expect that the 55 would also require a
heading reference to support its intercept feature.

It seems like a heading reference might improve the ability of an autopilot
to maintain course precisely or at least reduce any tendency to weave or
hunt while bracketing the course. Would anyone with direct experience like
to offer a comparison?

"Craig Prouse" wrote:
It seems like a heading reference might improve the ability of an
autopilot to maintain course precisely or at least reduce any
tendency to weave or hunt while bracketing the course. Would
anyone with direct experience like to offer a comparison?

I can only testify about my S-Tec 50, which is the same in this
respect as the 20 or 30. It works strictly off CDI deflection and rate
of turn. I don't think there is room for improvement. Using the GPS
CDI, once established on a track, it will maintain a xtrack error of
just a few feet without hunting. That is with the autopilot set to
"approach" sensitivity and the GPS CDI scale set to one mile.
--
Dan
C172RG at BFM

First of all, thank you for your responses.

I want to make sure I understand what you are saying. The AP only gets one
piece of information from the CDI... the equivalent of the left/right
deflection of the needle. It is not getting the OBS setting. This piece of
information comes from the heading bug. When tracking a radial, though, it
would seem the OBS setting will tell you the desired heading. Also, the
manual instructs you to always set the heading bug to the same value as the
OBS. Intercepting the course works because when you first select NAV and
the HDG message flashes you set the HDG bug to the OBS setting, then it
basically ignores the HDG for a while and either uses the current heading
(all angles intercept) or computes a 45 deg intercept. Again, if the AP had
the OBS setting and the current heading, it could figure this out. It seems
to me this may be a cost issue. The way it is now the AP only needs the
following inputs:

Current Heading
Heading Bug
CDI deflection

If it was able to follow a course from the CDI/OBS alone it would need an
additional input.

Thank you for all the responses.

martin


"Mike Rapoport" wrote in message
k.net...
By setting the heading bug and the OBS to the same course you are
mimicking
a HSI. The data out of the OBS is only course deviation, it doesn't
output
the course that is set on the dial. If the numbers on the OBS were
covered
up and I turned the knob to a randome course, you would have a difficult
time flying that course. You might eventually figure it out, but you also
might make a bunch of 90 degree intercepts first. By setting both, you
are
telling it what the desired course is (HDG) and the current deviation from
that course (CDI).

Life is simpler with an HSI which combines the DG and OBS onto one
instrument...highly recommended.


Mike
MU-2


"News" wrote in message
.. .
I have a question about some nuances of the Bendix/King KAP 140
autopilot.
Since the ink is still wet on my IFR rating (actually I have not even
gotten
the new one in the mail yet!) I am working on a personal "post
checkride"
syllabus. Part of this self training is to fly different routes on an
IFR
clearance in VMC using the autopilot. In all my VFR days I never
touched
the thing and was not allowed to use it during my IFR training. While I
am
serious about keeping my hand flying skills sharp, I also want to know
how
to use the autopilot. After reading the manufacturer's "Pilot's Guide"
cover to cover I noticed a rather odd procedure when using the AP in
conjunction with a DG. When you select NAV the AP temporarily flashes
HDG
to remind you to set the Heading Bug to the same course as set on the
OBS.
I do this and everything works fine but I want to understand WHY you do
this. What algorithm is the autopilot is using? How does it uses the
Heading bug info in conjunction with the OBS and CDI? I noticed that
once
I
am tracking a radial if I change the HDG bug the aircraft turns. My
mental
model has the autopilot just using the CDI deflection and OBS setting to
hold a course, but obviously I am missing something. If anyone can
point
me
to some more detailed information on how the AP uses the Heading Bug
while
in NAV or APR mode I would appreciate it.

thanks
Martin Van Ryswyk
PPSEL Instrument

Ross Magnaldo wrote:
Dave,

You have confused me even more!
If I am flying a radial with a cross wind, should the HDG bug be on the
radial or should it be on the estimated course made good with the wind
correction?

Sorry to confuse you, Ross.

Stating the disclaimer again: I've never flown with a KAP140, so my information
is based on autopilots that I *assume* operate similarly.

Set the heading bug on the desired radial. The track will be slightly off the
radial if you have a crosswind. This is usually good enough.

As a refinement, you can offset the heading bug in the direction into the wind,
and the aircraft will track closer to the desired radial.

Remove SHIRT to reply directly.

Dave

Dave Butler ) wrote:

Set the heading bug on the desired radial. The track will be slightly
off the radial if you have a crosswind. This is usually good enough.

True, in my experiences.

As a refinement, you can offset the heading bug in the direction into
the wind, and the aircraft will track closer to the desired radial.

With the KAP140 in this case, changing the heading bug even a few degrees
to match your current course (after the AP had been tracking the course)
directs the AP to turn in the direction you turned the heading bug, away
form the desired course. Left alone, the AP will soon turn back and re-
intercept the desired course.

This also occurs after the DG precesses and the pilot attempts to reset it
to the compass.


--
Peter












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Peter R. wrote:
Dave Butler ) wrote:


Set the heading bug on the desired radial. The track will be slightly
off the radial if you have a crosswind. This is usually good enough.


True, in my experiences.


As a refinement, you can offset the heading bug in the direction into
the wind, and the aircraft will track closer to the desired radial.


With the KAP140 in this case, changing the heading bug even a few degrees
to match your current course (after the AP had been tracking the course)

I assume you meant "...to match your current heading".

directs the AP to turn in the direction you turned the heading bug, away
form the desired course. Left alone, the AP will soon turn back and re-
intercept the desired course.

If you turned the heading bug in the direction to match your current *heading*,
and and the AP turns in the direction you turned the heading bug, That turn will
be *toward*, not away from the desired course.

Remove SHIRT to reply directly.

Dave

Dave Butler ) wrote:

I assume you meant "...to match your current heading".

Yes.

directs the AP to turn in the direction you turned the heading bug, away
form the desired course. Left alone, the AP will soon turn back and re-
intercept the desired course.

If you turned the heading bug in the direction to match your current *heading*,
and and the AP turns in the direction you turned the heading bug, That turn will
be *toward*, not away from the desired course.

No, that is not what I am saying. The AP is already tracking the current
course, which results in maintained heading. However, the heading bug
happens to be five degrees to either side of the current heading. This
could either be because of a crosswind or perhaps DG precession.

If the pilot moves the heading bug to match the current heading, which is
also the course being tracked by the AP, the AP momentarily turns *away*
from the desired course.

--
Peter












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I wrote:

snip
If the pilot moves the heading bug to match the current heading, which is
also the course being tracked by the AP, the AP momentarily turns *away*
from the desired course.

Whoops, left off a bit...

In my example, if the pilot moves the heading bug to the right to match the
desired heading, the KAP140 will momentarily turn to the right, off course.
If the pilot moves the heading bug to the left to place it over the desired
heading, the KAP140 momentarily turns to the left, again off course.

--
Peter












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"Peter R." wrote in message
...
Dave Butler ) wrote:

I assume you meant "...to match your current heading".

Yes.

directs the AP to turn in the direction you turned the heading bug,
away
form the desired course. Left alone, the AP will soon turn back and
re-
intercept the desired course.

If you turned the heading bug in the direction to match your current
*heading*,
and and the AP turns in the direction you turned the heading bug, That
turn will
be *toward*, not away from the desired course.

No, that is not what I am saying. The AP is already tracking the current
course, which results in maintained heading. However, the heading bug
happens to be five degrees to either side of the current heading. This
could either be because of a crosswind or perhaps DG precession.

If the pilot moves the heading bug to match the current heading, which is
also the course being tracked by the AP, the AP momentarily turns *away*
from the desired course.

Think "selected heading", as "selected course" is a different thing in
autopilot land. (gyro vs radio)

Peter ) wrote:

Peter R. wrote

No, that is not what I am saying. The AP is already tracking the current
course, which results in maintained heading. However, the heading bug
happens to be five degrees to either side of the current heading.

I take it you don't have an HSI - with an HSI it is the course pointer
(not the heading bug) which is set to the desired track. The heading
bug is ignored completely.

That's correct. I do not have experience flying behind an HSI. With about
500 hours in a C172, I am getting the itch to move up.


--
Peter












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